Vancomycin is an increasingly important antibiotic in treating resistant gram positive infections. We have for the first time evaluated its disposition in critically ill patients following multiple dosing. Using initial pharmacokinetic parameter estimates for its disposition in the young and elderly, the multi-dose data gathered in these patients were fit by a previously proposed 3-compartment model. Estimates obtained following kinetic analysis of these patients suggest that single dose of the drug may serve as a test dose to predict drug levels following multiple dosing. Vancomycin disposition in man, therefore, appears to be linear up to 20 mg/kg dosages. Three patients with renal insufficiency showed 30-60% reductions in the clearance of vancomycin and prolonged elimination half-lives. The study also revealed that renal (but not hepatic) insufficiency can cause significant alteration in its disposition which can be predicted from serial assessments of renal function.